Greenhouses have been used extensively for growing agricultural products including vegetables, fruits, flowers and other indoor plants where a narrow range of desired temperature is required for the viable biological processes of plants. When the interior temperature of the greenhouse is outside the range, the plants can suffer from irreversible loss or die. It is not critical, however, to control the relative humidity inside an agricultural greenhouse as long as sufficient moisture is supplied to the soil. It is undesirable to remove much moisture from an agricultural greenhouse because extra water would be provided to the soil or the plant would be dehydrated.
Wood drying is normally achieved by using a kiln apparatus, which is heated by steam or direct heating sources including wood or gas furnaces. A kiln apparatus consumes significant amount of fuel or electricity, and thus expensive to operate. Relative high temperature in a kiln apparatus can also cause internal stresses in a wood product, resulting in deformation or cracks. Wood can be air dried by putting wood under covered spaces with adequate air circulations. Air-drying requires extended drying time and is less controllable due to natural variation of weather conditions.